One of the advantages of having two frontally-placed eyes is that the central nervous system (CNS) can make use of the two slightly different views provided by the horizontal separation of the eyes. This provides for stereopsis which augments depth perception but also carries with it the problem of how to keep the images from the two retinas in correspondence to avoid double vision. In addition there are two modes in which depth information is processed. There is a pre-attentive mode in which novel stimuli attract our attention and stimulate eye movements to foveate the target and there is an attentive mode in which there is willful inspection of the acquired target. These modes are referred to as transient and sustained respectively.
On the sensory side, our laboratory is interested in how the visual system recognizes when the two images are in correspondence during transient (large brief disparity) and sustained (small prolonged disparity) presentations. The transient system is able to match very dissimilar shaped targets of unequal spatial frequency as long as they have equal equivalent signal strength (Edwards, Pope and Schor, 1997). In contrast, the sustained system requires a high degree of similarity between the two ocular images before they can be matched (Schor Wood and Ogawa, 1984).
On the motor side we need to adjust vergence and accommodation for objects of interest that are nearby, wherein the eyes move in the opposite directions and increase their optical power. Previous experiments have revealed that there are two vergence mechanisms; a transient mechanism that gets the eyes moving toward the target and a fusional mechanism that keeps the two eyes locked onto the target once the eyes are on target. The velocity of the transient responses of vergence and accommodation are enhanced when accompanied by saccades (Lott, Schor and Pope, 1997). In order for vergence to remain "locked in" the images in the two eyes need to be quite similar. We are currently examining how similar the two images need to be in order to initiate the transient vergence response (Schor, Edwards and Pope, 1997).
Binocular alignment is achieved with both retinal (visual) and extra-retinal (non-visual) stimuli. Even when one eye is occluded, both eyes aim accurately on objects located in almost any direction or distance in space (Schor,Maxwell & Stevenson, 1993). This accuracy is due to the plasticity of the oculomotor system and an adaptive calibration mechanisms that keeps the two eyes pointed in the correct direction despite changes that may occur either in the CNS or in the orbit during development or because of disease or injury (Maxwell and Schor 1996 & 1997; McCandless, Schor, & Maxwell 1996; McCandless and Schor 1997). We are currently studying the plasticity and synergy of vertical and torsional eye movements, particularly the cross-coupling between head position and eye position. Another classical coupling is between accommodation and convergence (Schor and Kotulak, 1986) that synchronizes focus and alignment of the eyes while performing near tasks. These adaptive manipulations have important consequences for the rehabilitation of paretic eye muscle disorders and for prediction of space-motion illness reported by pilots.